Positron Emission Tomography
Positron emission tomography (PET) is an imaging method for obtaining quantitative molecular and biochemical information of physiological processes in the body. The most common PET radiopharmaceutical in use today is [18F]-fluorodeoxyglucose ([18F]-FDG), a radiolabeled glucose molecule. PET imaging with [18F]-FDG allows to visualize glucose metabolism and has a broad range of clinical indications. Among positron emitters, 18F is the most widely used today in the clinical environment. Due to the increasing regulatory pressure, the radiopharmaceuticals are usually prepared today on single use components assembled in ready-to-use cassettes.
18F-Labeled Choline Analogues (Data Extracted from MICAD Database)
Choline is an important component of phospholipids in the cell membranes. Tissues with increased metabolism will lead to an increased uptake of choline. Choline is phosphorylated by choline kinases (CHK) to phosphorylcholine within cells, and, after several biosynthetic processes, finally is integrated into phospholipids (Zeisel S. H., Blusztajn J. K. Choline and human nutrition. Annu Rev Nutr. 1994; 14:269-96.). Because tumor cells have a high metabolic rate, choline uptake is high in order to keep up with the demands with the synthesis of phospholipids in their cellular membranes (Podo F. Tumour phospholipid metabolism. NMR Biomed. 1999; 12 (7):413-39.).
Positron emission tomography (PET) with [11C]choline has been reported to be useful for the detection and differential diagnosis of brain tumors, prostate cancer, lung cancer, and esophageal cancer (Hara T. [11C]-choline and 2-deoxy-2-[18F]fluoro-D-glucose in tumor imaging with positron emission tomography. Mol Imaging Biol. 2002; 4 (4):267-73.). However, [11C]choline has a high uptake in liver, kidney, and spleen. [18F]-labeled choline analog was initially synthesized as [18F]fluoroethylcholine to replace [11C]choline as a PET tracer due to the short physical half-life of 11C (Hara T., Yuasa M. Automated synthesis of fluorine-18 labeled choline analogue: 2-fluoroetheyl-dimethyl-2-oxyethylammonium. J Nucl Med. 1997; 38 Supplement:44P.). Although 18F has a longer half-life (110 min), [18F]fluoroethylcholine showed a rapid accumulation in the urinary bladder, rendering it less desirable for imaging prostate cancer and pelvic lymph nodes. Therefore, [18F]fluorocholine (FCH) was conceived to be a better biological analog than [18F]fluoroethylcholine (DeGrado T. R., Coleman R. E., Wang S., Baldwin S. W., Orr M. D., Robertson C. N., Polascik T. J., Price D. T. Synthesis and evaluation of 18F-labeled choline as an oncologic tracer for positron emission tomography: initial findings in prostate cancer. Cancer Res. 2001; 61 (1):110-7.). FCH PET studies showed high uptake in malignancies in patients with prostate cancer, breast carcinoma, and brain tumors (DeGrado T. R., Baldwin S. W., Wang S., Orr M. D., Liao R. P., Friedman H. S., Reiman R., Price D. T., Coleman R. E. Synthesis and evaluation of (18)F-labeled choline analogs as oncologic PET tracers. J Nucl Med. 2001; 42 (12):1805-14; Hara T., Kondo T., Hara T., Kosaka N. Use of 18F-choline and 11C-choline as contrast agents in positron emission tomography imaging-guided stereotactic biopsy sampling of gliomas. J Neurosurg. 2003; 99 (3):474-9.).
Synthesis Methods
Standard Method Involving Gaseous Intermediates
[18F]Choline is generally synthesized from [18F]fluorobromomethane and dimethylethanolamine (DMEA) with a radiochemical purity greater than 98% and a radiochemical yield (not corrected for decay) for the synthesis and purification was approximately 20-40%. (DeGrado T. R., Coleman R. E., Wang S., Baldwin S. W., Orr M. D., Robertson C. N., Polascik T. J., Price D. T. Synthesis and evaluation of 18F-labeled choline as an oncologic tracer for positron emission tomography: initial findings in prostate cancer. Cancer Res. 2001; 61 (1):110-7.) In D. Kryza et al (Fully automated [18F]fluorocholine synthesis in the TracerLab MXFDG Coincidence synthesizer. Nucl. Med. Biol. 35 (2), 2008: 255-260), [18F]Fluorocholine was prepared by N-alkylation of DMAE with [18F]fluorobromomethane (BrCH2F), followed by purification on a CM cartridge.
Another automated method of FCH synthesis was achieved through the formation of [18F]fluoromethyl triflate and the reaction of [18F]fluoromethyl triflate with DMEA on a Sep-Pak column. The total time required for obtaining the finished chemical was 30 min. The radiochemical yield (decay corrected) was 80% with the radiochemical purity and chemical purity of >98%. (Iwata R., Pascali C., Bogni A., Furumoto S., Terasaki K., Yanai K. [18F]fluoromethyl triflate, a novel and reactive [18F]fluoromethylating agent: preparation and application to the on-column preparation of [18F]fluorocholine. Appl Radiat Isot. 2002; 57 (3):347-52.)
Alternative Methods Using Ditosylates as Precursor
G. Smith et al (Nucl Med Biol. 2011 January; 38 (1):39-51) have compared the synthesis of [18F]fluorocholine, by the alkylation of the relevant precursor, i.e. DMEA, with [18F]fluorobromomethane or [18F]fluoromethyl tosylate. Alkylation with [18F]fluoromethyl tosylate proved to be the most reliable radiosynthetic route.
In WO 2005/009928, J. Lim has shown the preparation of [18F] FCH in a 2-steps reaction: fluorination of ditosylmethane with [18F] fluoride followed by an alkylation reaction with [18F]fluoromethane tosylate and dimethylethanolamine using a [18F]FCH was purified using a Silica Sep-Pak column (Catalog No. WAT023537, Waters Corporation, Milford, Mass.). The column was washed with ethanol and water to remove all impurities and [18F] FCH was eluted with 2% acetic acid. The acetic acid was removed using an AG 4-X4 weakly basic ion-exchange resin column (143-3341, Bio-Rad Laboratories, Inc., Hercules, Calif.).
It was shown by G. Pascali et al (Dose-on-demand of diverse 18F-fluorocholine derivatives through a two-step microfluidic approach. Nucl. Med. Biol. 38 (5), 2011: 637-644) that the radiolabeling step results in the formation of two [18F]-labeled species among which [18F]fluoromethane tosylate. Despite this method uses non-gaseous intermediates, no purification method is carried out or proposed in order to eliminate impurities and side products resulting from this reaction path. Beyerlein et al have evidenced that the second compound is [18F]-labeled Tosyl Fluoride (Beyerlein et al, J. Label Compd. Radiopharm. Vol 56 (14), 2013).
It was recently demonstrated (Rodnick et al, Applied Radiation and Isotopes 78 (2013)26-32) the presence of cold impurities generated during the quaternization reaction, e.g. the reaction of methane ditosylate with DMAE, especially when doing one-pot labeling and quaternization, these impurities being present in the final product. These impurities have to be eliminated and up to recently, the only method to perform a reliable purification was high pressure liquid chromatography.